Process for producing filled porous PTFE products

ABSTRACT

This invention provides a tetrafluoroethylene polymer in a porous form which has an amorphous content exceeding about 5% and which has a micro-structure characterized by nodes interconnected by fibrils. The material has high porosity and high strength. It can be used to produce all kinds of shaped articles such as films, tubes, rods, and continuous filaments. Laminations can be employed and impregnation and bonding can readily be used to produce a large variety of articles. Compressed articles of very high strength can also be produced from these porous forms.

This is a division of application Ser. No. 376,188 filed July 3, 1973now Pat. No. 3,953,566 which in turn is a continuation of applicationSer. No. 39,753, filed May 21, 1970, now abandoned.

Tetrafluoroethylene polymers and, in particular, poly(tetrafluoroethylene) are gaining more and more uses because of theirchemical inertness and desirable physical properties such aswater-repellency and electrical insulating abilities. In one very largearea, the field of porous articles, their use has been substantiallyblocked by the very considerable difficulty of making an article porousand keeping it so and providing it with adequate stregnth. Complicated,expensive processes have been devised such as adding a filler to thepolymer prior to shaping and then removing the filler after shaping, forexample, by leaching it out of the shaped article with a solvent or bymelting or burning it out. Not only are the process steps time consumingbut the cost of such processes make them unattractive commercially.

Therefore, an objective of this invention is the provision of economicalprocesses for producing highly porous materials from tetrafluoroethylenepolymers. A further aim is to provide such processes which impart veryhigh strengths to the resultant products. A still further purpose isproviding the products themselves and, in particular, products frompoly(tetrafluoroethylene) which are highly porous and have highstrengths. Also, dense products of polytetrafluoroethylene are producedthat have extremely high strength. These and other objectives appearhereinafter.

The invention described herein provides products of atetrafluoroethylene polymer which have outstanding combinations of highporosity and high strength. In this regard they not only exceedpreviously available fluorocarbon polymeric products, but are uniqueamong porous plastic materials. The porous structure produced by theprocesses of this invention is permeable and can be laminated,impregnated, and bonded with other materials to provide compositestructures having novel and unique properties.

The objectives of this invention are accomplished by a process involvingexpanding paste-formed products of a tetrafluoroethylene polymer to makethem both porous and stronger, and subsequently heat treating them toincrease their strength further while retaining a porous structure.Paste-forming techniques are used to convert the polymer in paste formto a shaped article which is then expanded, after removing thelubricant, by stretching it in one or more directions; and while it isheld in its stretched condition it is heated to at least 327° C afterwhich it is cooled. The porosity that is produced by the expansion isretained for there is little or no coalescence or shrinking uponreleasing the cooled, final article. The description and the examplesbelow further describe the processes and the products of this invention.

Paste-forming of dispersion polymerized poly(tetrafluoroethylene) iswell known commercially. Extrusions of various cross-sectional shapessuch as tubes, rods and tapes are commonly obtained from a variety oftetrafluoroethylene resins, and other paste-forming operations such ascalendering and molding are practiced commercially. The steps inpaste-forming processes include mixing the resin with a lubricant suchas odorless mineral spirits and carrying out forming steps in which theresin is subjected to shear, thus making the shaped articles cohesive.The lubricant is removed from the extruded shape usually by drying. Inusual practice this unsintered product is heated above the polymer'smelting point, generally about 327° C., causing it to sinter or coalesceinto an essentially impermeable structure. However, it is the unsinteredproduct that is the precursor of the invention herein.

In this invention it has been found that such paste-formed, dried,unsintered shapes can be expanded by stretching them in one or moredirections under certain conditions so that they become substantiallymuch more porous and stronger. This phenomenon of expansion withincrease in strength occurs with certain preferred tetrafluoroethyleneresins and within preferred ranges of rate of stretching and preferredranges of temperature. The preferred temperature range is from 35° C to327° C. At the lower temperatures within this range it has been foundthat there is a maximum rate of expansion beyond which fracture occurs,as well as a lower limit beneath which fracture also occurs or whereweak materials are obtained. The lower limit is of much more practicalsignificance. At high temperatures within this range, only the lowerlimit of rate has been detected. The lower limit of expansion ratesinteract with temperature in a roughly logarithmic fashion, being muchhigher at higher temperatures. Most, but not all, of the desirableproducts of this invention are obtained when expansion is carried out atthe higher temperatures within the range of 35° C to 327° C. The balanceof orientation in the extruded shape also affects the relationshipbetween the proper range of rates and temperature. It is found that someresins are much more suitable for the expansion process than others,since they can be processed over a wider range of rate and temperatureand still produce useful products. The primary requisite of a suitableresin is a very high degree of crystallinity, preferably in the range of98% or above, and correspondingly low amorphous content. It has beenfound that techniques for increasing the crystallinity, such asannealing at high temperatures just below the melt point, improve theperformance of the resin in the expansion process. Copolymers oftetrafluoroethylene, which have defects in the crystalline structurethat introduce a higher amorphous content, do not work as well in thisinvention as homopolymers. However, it is found, for example, thatresins which contain less than 0.2% of hexafluoropropylene as aco-monomer can be made to work in this invention by going to very highrates of expansion at high temperatures just below the melt point.

The porous microstructure of the expanded material is affected by thetemperature and the rate at which it is expanded. The structure consistsof nodes interconnected by very small fibrils. In the case of uniaxialexpansion the nodes are elongated, the longer axis of a node beingoriented perpendicular to the direction of expansion. The fibrils whichinterconnect the nodes are oriented parallel to the direction ofexpansion. These fibrils appear to be characteristically wide and thinin cross-section, the maximum width being equal to about 0.1 micron(1000 angstroms) which is the diameter of the crystalline particles. Theminimum width may be one or two molecular diameters or in the range of 5or 10 angstroms. The nodes may vary in size from about 400 microns toless than a micron, depending on the conditions used in the expansion.Products which have been expanded at high temperatures and high rateshave a more homogeneous structure, i.e. they have smaller, more closelyspaced nodes and these nodes are interconnected with a greater number offibrils. These products are also found to have much greater strength.

It should be noted that during the expansion process a tremendousincrease in strength is introduced into the structure, for while theporosity increases the strength actually increases, so there is oftengreater than a tenfold increase in strength of the polymeric matrix. Inpatent application Ser. No. 863,446, filed Oct. 3, 1969, a process isdescribed for expanding unsintered poly(tetrafluoroethylene) sheet, rodsand shapes to give low density but low strength products. However, Ihave discovered that by performing the stretching at a very high rate, asurprising increase in strength is obtained. Although most materialsfracture when subjected to a high rate of strain, highly crystallinepoly(tetrafluoroethylene) withstands this treatment without breaking. Bydefinition, the tensile strength of a material is the maximum tensilestress, expressed in force per unit cross sectional area of thespecimen, which the specimen will withstand without breaking (see, forexample, The American Society for Testing and Materials. "1970 AnnualBook of ASTM Standards - Part 24", at p. 41). For porous materials, thecross sectional area of solid polymer within the polymeric matrix is notthe cross sectional area of the porous specimen, but is equivalent tothe cross sectional area of the porous specimen multiplied by thefraction of solid polymer within that cross section. This fraction ofsolid polymer within the cross section is equivalent to the ratio of thespecific gravity of the porous specimen itself divided by the specificgravity of the solid polymeric material which makes up the porousmatrix. Thus, to compute matrix tensile strength of a porous specimen,one divides the maximum force required to break the sample by the crosssectional area of the porous sample, and then multiplies this quantityby the ratio of the specific gravity of the solid polymer divided by thespecific gravity of the porous specimen. Equivalently, the matrixtensile strength is obtained by multiplying the tensile strengthcomputed according to the above definition by the ratio of the specificgravities of the solid polymer to the porous product. In the exampleswhich follow, both tensile strength and matrix tensile strength areshown, computed according to the above method, the lowest matrixstrength measured being above 7300 p.s.i. In other words, the productsshown herein all have matrix strengths of above about 7300 p.s.i.

When the expanded products are heated to above the lowest crystallinemelting point of the poly(tetrafluoroethylene), disorder begins to occurin the geometric order of the crystallites and the crystallinitydecreases, with concomitant increase in the amorphous content of thepolymer, typically to 10% or more. These amorphous regions within thecrystalline structure appear to greatly inhibit slippage along thecrystalline axis of the crystallite and appear to lock fibrils andcrystallites so that they resist slippage under stress. Therefore, theheat treatment may be considered an amorphous locking process. Theimportant aspect of amorphous locking is that there be an increase inamorphous content, regardless of the crystallinity of the startingresin. Whatever the explanation, the heat treatment above 327° C. causesa surprising increase in strength, often doubling that of theunheat-treated material.

Because the upper melting range of poly(tetrafluoroethylene) polymer (aspolymerized) is about 345° C, the heat treatment appears to be moreeffective above this temperature, although lower temperatures areequivalent if the exposure time is long enough. The optimum heattreating temperature is in the range of 350° C to 370° C and the heatingperiods required may range from about 5 seconds to about one hour. Themicrostructure of the expanded product is not substantially changed bythe amorphous locking step. However, if the amorphous locking is carriedout at too high a temperature for too long a time, the microstructuremay become coarse as the nodes increase in size and the fibrils rupture,and in this case there is a noticeable deterioration in strength, butthis presents no problem since one can very readily determine theoptimum time and temperature for the given tetrafluoroethylene polymerbeing processed. Temperatures above about 390° C may cause thisdisintegration and loss of strength in less than one minute. In heattreating films it is essential that they be held so they cannot retractduring the amorphous locking process. It is surprising that the expandedstructures of this invention do not coalesce during the heat treatmentto form high density products. If unexpanded films, having a density ofabout 1.5 gm/cm³ are so heated, they coalesce to form an essentiallyvoid-free material having a room temperature density of about 2.15gm/cm³. Very little increase in density occurs when the products belowabout 1.00 gm/cm³ density are heated above the 327° C temperature.

The increase in strength of the polymer matrix is dependent upon thestrength of the extruded material before expansion, the degree ofcrystallinity of the polymer, the rate and temperature at which theexpansion is performed, and amorphous locking. When all these factorsare employed to maximize the strength of the material, tensile strengthsof 10,000 psi and above, with porosity of 90% or more are obtained. Inthese cases the polymeric matrix has strengths in excess of 100,000 psi.In contrast, the maximum tensile strength of conventional extruded ormolded poly(tetrafluoroethylene) after sintering is generally consideredto be about 3,000 psi, and for conventional extruded and calenderedpoly(tetrafluoroethylene) tape which has been sintered the maximum isabout 5,100 psi.

Before describing examples of processes and products within thisinvention, a further description of the properties of expanded,amorphous-locked tetrafluoroethylene polymers will be helpful. Asindicated above, some of the properties of these expanded, amorphouslylocked polymers are substantially different from the correspondingproperties of conventional extruded or molded tetrafluoroethylenepolymers. As a result of these differences, expanded, amorphously lockedmaterials are useful in many applications where extruded or moldedmaterials cannot be used.

These expanded, amorphous-locked materials have permeabilities to gases,and to liquids in some cases, which are much higher than thecorresponding permeabilities of conventional molded or extrudedpoly(tetrafluoroethylene). The permeability to nitrogen of conventionalpoly(tetrafluoroethylene) film is reported in The Journal of Teflon,Jan. - Feb. 1970 (du Pont) at page 10 to be about 1 × 10⁻¹⁰ metricunits.

In comparison, expanded, amorphous-locked films of this invention havepermeabilities to nitrogen from about 1 × 10⁻⁸ to 1 × 10⁻¹ metric units.These higher permeabilities are consistent with the lower densities andhigher porosities of the expanded, amorphous-locked films, compared withconventional films. Furthermore, by controlling the degree of expansionand the amorphous-locking conditions used, it is possible to maketetrafluoroethylene polymeric materials having any desired permeabilitywithin the range listed above. These permeability differences are dueprimarily to differences in pore sizes within the materials.

Also, permeabilities to liquids of the expanded, amorphous-lockedmaterials described herein are higher, in an analogous way, thancorresponding permeabilities to liquids of the conventional materials.

As a result of the ability of the expanded, amorphous-locked materialsdescribed herein to transmit fluids as described, these materials areuseful as filtering membranes to separate solid materials from gases andfrom liquids. For optimum filtering rates, relatively low-permeability,small-pore size membranes are used to filter out small solid particles,and high-permeability, large-pore size membranes are used to filter outlarge solid particles.

Also, the expanded, amorphous-locked materials described herein areuseful as semi-permeable membranes for separating wetting fluids fromnon-wetting fluids. For example, a gas-saturated membrane in contactwith water and gas will transmit the gas, the wetting phase, asdescribed above. But it will not transmit the water, the non-wettingphase, as long as the pressure in the water phase does not exceed thewater entry pressure for that particular combination of membrane andfluids.

One factor which influences entry pressure of a non-wetting fluid into aporous material is the size of the pores. Since the size of the pores inthe expanded, amorphous-locked materials described here can be and arecontrolled by the conditions used in the expanding and amorphous-lockingoperations, these materials are very useful, under a wide variety ofconditions, as semi-permeable membranes.

The usefulness of the materials covered by this invention as filteringmembranes for separating solids from fluids or as semi-permeablemembranes for separating immiscible fluids from each other is enhancedby the following well-known highly desirable properties oftetrafluoroethylene polymeric materials: (1) outstanding chemicalinertness and (2) resistance to undesirable physical changes over a widetemperature range.

The expanded, amorphous-locked material of this invention can be bondedto other materials and to itself much more readily than conventionalpoly(tetrafluoroethylene) products can. This is true because bondingagents are able to penetrate a significant distance into the porenetwork of expanded, amorphous-locked material, and, after hardening,they become locked in place. In constrast, there is negligiblepenetration of bonding agents into conventional tetrafluoroethylenepolymers, and this, coupled with the general non-bonding nature of lowenergy surfaces make bonding difficult.

Certain other properties of expanded, amorphous-lockedpoly(tetrafluoroethylene) materials are better than the correspondingproperties of conventional extruded or molded poly(tetrafluoroethylene)products, making the former materials more useful in many applicationsthan the latter. The termal conductivity of molded conventionalpoly(tetrafluoroethylene) is about 1.7 Btu/hu/sq.ft./°F./in. while thatof the expanded, amorphous-locked polymer ranges from about one-tenth toabout one-half that value. In line with this, the more highly expandedmaterials of this invention have proven to be useful thermal insulators.

Similarly, expanded, amorphous-locked poly(tetrafluoroethylene) hasshown an advantage over the conventional homopolymer as an electricalinsulator in coaxial cables. The lower dielectric constant of theformer, about 1.2 to 1.8, as compared with 2.2 for conventional polymer,permits smaller and lighter cables to be contructed by using the former.Many applications in which weightsaving (i.e. use of low densitymaterial) is an advantage can benefit by using the expanded,amorphous-locked polymers described herein in preference to conventionalhigh density tetrafluoroethylene polymers.

This invention will be further understood by reference to the examplesgiven below and to the drawings, all of which are given for illustrativepurposes only and are not limitative, the drawings being:

FIG. 1 is a plan view of a section of an expanded, amorphously lockedtetrafluoroethylene polymer as seen under a microscope; and

FIG. 2 is a diagrammatical view of an apparatus that may be used in theprocess of this invention to produce the expanded, amorphously-lockedstructures.

As shown in FIG. 1, the expanded, amorphously locked, porous material 10of this invention comprises a large plurality of nodes 11 which areoriented perpendicularly to the direction in which the expansion waseffected. These nodes, on the average about 50 microns in size andfairly irregular in shape, lie closely together and in many instancesappear to touch at points. A given node is connected to adjacent ornearby nodes by fibrils 12 which vary in length from 5 to 500 micronsdepending upon the amount of expansion. While FIG. 1 shows a uniaxialexpansion effect, it will be appreciated that with expansion biaxiallyand with expansion in all directions, similar fibril formation occurs insaid directions with the production of spider-web-like or cross-linkedconfigurations and attendant increases in strength. The porosity alsoincreases as the voids or spaces between the polymeric nodes and fibrilsbecome more numerous and larger in size.

The apparatus shown in FIG. 2 is described below in Example 5.

EXAMPLE 1 Expansion of Rods

A cylindrical rod of 5/32 inch diameter was made by extruding a paste of"Teflon" 6A resin containing 130 cc/lb. of mineral spirits as anextrusion aid, at a reduction ratio of 370 (the resin being obtainablefrom E.I. du Pont de Nemours & Co., Inc.). The volatile extrusion aidwas removed by drying, the resultant rod having a specific gravity of1.63, a tensile strength of 531 psi, and an elongation of 183% (A. S. T.M. test method). The amorphous content of the "Teflon" 6A resin and theunsintered rod were determined using the infra-red method described byMoynihan, R. E. "IR Studies on Polytetrafluoroethylene", J. Am. Chem.Soc. 81, 1045-1050 (1959), and found to be 1.5%.

An apparatus was devised so that samples of the rod could be stretchedvarious amounts at controlled rates and controlled temperatures. Theapparatus consisted of two clamps for holding the rod, one clamp beingheld fixed within an oven while the other clamp was attached to a wireleading outside the oven to a rack-and-pinion pulling device driven by avariable speed motor. After the sample had been expanded by stretchingat the given controlled temperature, the oven temperature was raised to370° C for ten minutes while the samples were held clamped in theirextended condition. In some cases the samples broke during the expansionstep and this is noted in tables below. The term "broke" refers to thefact that the particular sample being tested broke under the conditionsgiven as an attempt was being made to stretch it to the final elongationgiven; the precise percentage of elongation at which the given samplebroke is not given.

As can be seen in Table 1A, all samples were successfully expanded to aporosity of about 68% under the conditions of temperature and rate ofstretch shown. Table 1B shows that samples at the lower values oftemperature and rate could not be expanded by stretching 550%, while therest of the samples were successfully expanded to a porosity of about84%. Table 1C shows that only two samples were successfully expandedwhen the stretch was 1500%. These samples were obtained at the highestvalues of rate and temperature and had a porosity of about 96%.

                                      TABLES 1A, 1B and 1C                        __________________________________________________________________________    Table 1A: Percent Stretch = 200                                                       Rate of Rate of Rate of Rate of                                       Temperature                                                                           Stretch Stretch Stretch Stretch                                       ° F                                                                            30%/sec.                                                                              100%/sec.                                                                             1000%/sec.                                                                            5000%/sec.                                    __________________________________________________________________________    200     67% porosity                                                                          67% porosity                                                                          67% porosity                                                                          66% porosity                                  400     66% porosity                                                                          68% porosity                                                                          67% porosity                                                                          66% porosity                                  600     66% porosity                                                                          66% porosity                                                                          67% porosity                                                                          68% porosity                                  Table 1B: Percent Stretch = 550                                                       Rate of Rate of Rate of Rate of                                       Temperature                                                                           Stretch Stretch Stretch Stretch                                       ° F                                                                            30%/sec.                                                                              100%/sec.                                                                             1000%/sec.                                                                            5000%/sec.                                    __________________________________________________________________________    200     broke   broke   broke   broke                                         400     broke   84% porosity                                                                          85% porosity                                                                          85% porosity                                  600     broke   84% porosity                                                                          84% porosity                                                                          83% porosity                                  Table 1C: Percent Stretch = 1500                                                      Rate of Rate of Rate of Rate of                                       Temperature                                                                           Stretch Stretch Stretch Stretch                                       ° F                                                                            30%/sec.                                                                              100%/sec.                                                                             1000%/sec.                                                                            5000%/sec.                                    __________________________________________________________________________    200     broke   broke   broke   broke                                         400     broke   broke   broke   broke                                         600     broke   broke   96% porosity                                                                          96% porosity                                  __________________________________________________________________________

This example illustrates that the most highly expanded products of thisinvention are obtained when the expansion is carried out at hightemperatures and high rates of stretch. The amorphous content of theserods was found to be 24%.

EXAMPLE 2 Expansion of Rods

Rods 5/32 inch in diameter were manufactured under conditions similar toExample 1, except that "Teflon" 6C resin was used, this also beingobtainable from said du Pont company. The amorphous content of the"Teflon" 6C resin and the unsintered rod were found to be 3.5%. Whileeffective expansion was not obtained under the conditions of Example 1,at very much higher rates of expansion, expansion within this inventiondid occur:

                  TABLE 2                                                         ______________________________________                                        Percent Stretch = 550                                                         Temperature                                                                            Rate of Stretch                                                                           Rate of Stretch                                                                           Rate of Stretch                              ° F                                                                             5,000%/sec. 10,000%/sec.                                                                              40,000%/sec.                                 ______________________________________                                        200      broke       broke       broke                                        400      broke       broke       68% porosity                                 600      broke       broke       68% porosity                                 ______________________________________                                    

Amorphously locking the porous products obtained applying the40,000%/sec. rate of expansion was effected and the microstructures ofthe products conformed to such as shown in FIG. 1. The amorphous contentafter heat treatment at 370° C was 35%.

EXAMPLE 3 Expansion of Films

The following experiments were performed using a pantograph, which is amachine capable of stretching films biaxially or uniaxially over a rangeof rates and temperatures. The pantograph used in these experiments wascapable of stretching 4 inches × 4 inches samples of film to 16 inches ×16 inches. The 4 inches × 4 inches film was gripped on each side by 13actuated clamps, which moved apart uniformly on a scissor mechanism. Thefilm was heated by hot air flow above and below.

A sample of film 6 inches wide, 0.036 inch thick, and of continuouslength was produced by the paste extrusion process from "Teflon" 6Apoly(tetrafluoroethylene) using 105 cc of mineral spirits per pound ofresin as an extrusion aid. After removing the extrusion aid by drying,the unsintered film was found to have the following properties: specificgravity of 1.65, longitudinal tensile strength of 300 psi and transversetensile strength of 250 psi.

Ex. 3A 4 inches by 4 inches sample of this film was conditioned forapproximately 4 minutes at 225° C in the pantograph and then stretchedbiaxially at a rate of 500%/sec. in each direction to a size of 16inches × 16 inches. The temperature of the film was then raised to 370°C for 5 minutes while held clamped in the extended condition. The filmwas then cooled to ambient temperature and the following properties werefound: specific gravity of 0.15, longitudinal tensile strength of 2,500psi (a matrix tensile strength of 36,700 psi) and transverse tensilestrength of 2,230 psi.

Ex. 3(b): A sample was prepared in all ways similar to Example 3(a)except that it was stretched in the pantograph at the lower rate of55%/sec. The resulting film was still cohesive but was found to haveweak areas, and a non-uniform appearance.

Ex. 3(c): A sample was prepared in all ways similar to Example 3(a)except that it was stretched at the even lower rate of 5%/sec. The filmdid not expand, but ruptured.

Ex. 3(d): A sample was prepared in all ways similar to Example 3(a)except that the temperature during expansion was 50° C. This film didnot expand, but ruptured.

Ex. 3(e): A sample of paste-extruded film was taken before removal ofthe extrusion aid and calendered to a thickness of 0.0043 inch. Thephysical properties of the film were measured: specific gravity of 1.60;longitudinal tensile strength of 2,200 psi, and transverse tensilestrength of 270 psi.

Samples of this film were stretch on the pantograph. The results aresummarized in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Example 3(e)                                                                            Expansion Rate                                                                          Expansion Rate                                                      In Longitudinal                                                                         In Transverse                                             Temperature                                                                             Direction Direction                                                 ° C                                                                              (%/sec.)  (%/sec.)  Result                                          __________________________________________________________________________    225       500       500       Ruptured                                        225       500       0         Long. Tensile = 3900 psi (a matrix tensile                                    strength of 12,800 psi)                                                       Trans. Tensile = 1150 psi                                                     Spec. Gravity = 0.70                            225       0         500       Ruptured                                        50        500       500       Ruptured                                        50        500       0         Ruptured                                        50        0         500       Long. Tensile = 2400 psi (a matrix tensile                                    strength of 7,360 psi)                                                        Trans. Tensile = 2700 psi                                                     Spec. Gravity -- 0.75                           225       5         5         Ruptured                                        225       5         0           "                                             225       0         5           "                                             50        5         5           "                                             50        5         0           "                                             50        0         5           "                                             __________________________________________________________________________

From the tabulated results, it can be seen that the film respondeddifferently depending on which axis was stretched but that at the lowrates rupture occurred irrespective of the direction of expansion.

EXAMPLE 3(f) Expanded Films Made By Biaxial Stretching

Another 4 inch × 4 inch sample of film of the type described in thesecond paragraph of Example 3 above was stretched in the pantographmachine. In this case, the film was stretched simultaneously in twodirections at right angles to each other, 100% in each direction. Thus,the surface area of the strectched film was four times the surface areaof the original film.

The film temperature was about 300° C during the stretching operation.Linear stretching rates of about 400% per second in each dimension wereused.

With the expanded film still in tension (stretcher clamps still holdingthe stretched film), hot air was circulated over the film such that thefilm temperature was about 360° C for five minutes. This causedamorphous locking within the film.

Finally, with the stretcher clamps still holding the film, the film wascooled rapidly to room temperature by blowing cold air against it. Thecooled film, which was then removed from the clamps, was the desiredexpanded, amorphous-locked film.

Properties of the original unexpanded film and of the final expanded,amorphous-locked film, which show the advantage of this invention, arelisted below.

                                      TABLE 4                                     __________________________________________________________________________                       Original   Expanded                                        Property          Unexpanded Film                                                                         Amorphous-Locked Film                             __________________________________________________________________________    Film Length, relative units                                                                        1       1.9                                              Film Width, relative units                                                                         1       2.0                                              Film Thickness, mils                                                                              36.0    31.5                                              Specific Gravity     1.65    0.45                                             Long, Tensile Strength, psi                                                                       300     1900 (matrix tensile strength                                                   of a 290 psi)                                   Transverse Tensile Strength, psi                                                                  250     1760                                              Permeability to air, metric units                                                                  4 × 10.sup.31 5                                                                 6 × 10.sup.-3                              __________________________________________________________________________

EXAMPLE 4 Expansion of Filled Films

The "Teflon" 6A resin, identified above, was blended with a commerciallyavailable asbestos powder in proportion of four parts by weight resin toone part asbestos. The mixture was lubricated with 115 cc of odorlessmineral spirits per pound of mixture and extruded into a film 6 incheswide, 0.036 inches thick, and of continuous length. The film was thencalendered to 0.008 inch thickness and the extrusion aid removed bydrying. The properties were measured and found to be as follows:specific gravity of 1.44, longitudinal tensile strength of 1,000 psi;and transverse tensile strength of 205 psi.

A 4 inch × 4 inch sample was mounted in the pantograph described aboveand stretched at a rate of 500%/sec. at a temperature of 225° C and tothree times its original length in the longitudinal direction while nostretch was applied in the transverse direction. A sample of the filmwas tested and found to have the following properties: specific gravityof 0.82, longitudinal tensile strength of 1500 psi, and transversetensile strength of 145 psi. The remainder of the sample was placed inclamps to restrain it from shrinking, heated to 370° C for five minutes,and then cooled to room temperature. The following properties weremeasured on this sample: specific gravity of 0.95, longitudinal tensilestrength of 2,900 psi, and transverse tensile strength of 750 psi.

The heat treating of the film substantially increased its tensilestrength as can be seen from the above values, and had a very littleeffect on specific gravity.

EXAMPLE 5 Manufacture of Continuous Lengths of Porous Film

A machine was constructed for manufacturing long lengths of expandedfilm. As can be seen in FIG. 2, unsintered film 13 from the pasteextrusion process is fed to the machine from roll 14 onto heated roll 15where the film is preheated to the temperature at which it will beexpanded. Rolls 15 and 16 are of the same diameter and are connectedthrough a gear box so their relative rates of rotation can be changed.Roll 16 can be driven faster than roll 15 so that the film is stretchedin the gap "A" between the rolls making the film expand. The differencein speed determines the amount of stretch and thus the amount ofexpansion. For example, when roll 16 is driven twice as fast as roll 15,the film is expanded approximately 100% because, unlike other films, theunsintered poly(tetrafluoroethylene) film changes very little inthickness or width and the length increases by 100%. The increase involume is due to an increase of porosity and a corresponding decrease ofspecific gravity. The relative positions of rolls 15 and 16 areadjustable so that the "A" between them can be varied. This allows oneto control the rate of expansion. For example, when the gap distance ishalved, the rate of expansion is doubled. It should be noted that therate of expansion is also affected by the rate at which film is fed intothe machine. Roll 16 is maintained at the same temperature as roll 15.Expanded film leaves roll 16 and goes onto roll 17 (running at the sameperipheral speed) which is hot, and where the film is heated toapproximately 370° C so that amorphous locking will occur. The residencetime of film on this roll is controlled by the position of roll 18,which can be removed around the periphery of roll 17. Roll 19 is watercooled to reduce the temperature of the film as it passes thereoverbefore being wound up on roll 20. Thus, with this machine one is able tocontrol the three important variables necessary for expandingtetrafluoroethylene polymer film, i.e. the temperature, the rate ofexpansion, and the amount of expansion.

Three runs on this apparatus are given in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                 " Fluon" CD-1 (obtainable   "Teflon" 6A preheated                             from Imperial Chemical      3 hrs. at 300+ C. prior              Resin:       Industries, Ltd.)                                                                           "Teflon" 6A   to paste extrusion                   __________________________________________________________________________    Properties of starting film:                                                               (a)           (b)           (c)                                  Thickness of Film                                                                          0.0050"       0.0050"       0.0050"                              Density, gm/cm.sup.3                                                                       1.47          1.52          1.54                                 Longitudinal tensile, psi                                                                  1600          1900          2650                                 Transverse tensile, psi                                                                    200           250           350                                  Processing conditions:                                                        Tape feed rates:                                                              roll 14 to roll 15                                                                         30 ft./min.   30 ft./min.   30 ft./min.                          Temp., Rolls 15 and 16                                                                     300° C.                                                                              300° C.                                                                              300° C.                       Roll speed ratio:                                                             roll 15:roll 16                                                                            1:2.87        1:2.87        1:2.87                               Temp. for roll 17                                                                          370° C.                                                                              370° C.                                                                              370° C.                       Dwell time on roll 17                                                                      3 sec.        3 sec.        3 sec.                               Properties of Final Film:                                                     Thickness    0.0047"       0.0048"       0.0046"                              Density, gm/cm.sup.3                                                                       0.66          0.67          0.73                                 Long. Tensile, psi                                                                         2850(matrix tensile strength                                                                4000(matrix tensile strength                                                                8950(matrix tensile                               of 9,500 psi) of 13,100 psi)                                                                              strength of 27,000 psi)              Transverse tensile, psi                                                                    850           1050          1300                                 __________________________________________________________________________

EXAMPLE 6 Expanded, Amorphously Locked, Laminated Film Made From TwoLayers of Expanded Film

Using the tape expanding machine illustrated in FIG. 2 but with theamorphous-locking roll 17 set at 300° C., a temperature belowamorphous-locking temperature, a sample of expanded, "Teflon" 6Apoly(tetrafluoroethylene) film was made. This film had a specificgravity of 0.60 longitudinal tensile strength of 1900 psi (a matrixtensile strength of 7,300 psi), transverse tensile strength of 110 psi,and a thickness of 3.5 mils.

Two sections of this film, at right angles to each other and one on topof the other, were clamped to a rigid frame which secured all four edgesof the sandwich and pushed one film lightly against the other over thewhole area of contact. This assembly was given an amorphous-lockingtreatment by heating it at about 370° C. for 7 minutes. Then the wholeassembly was rapidly cooled with a stream of cold air, and clamps werereleased yielding the desired one-piece laminated film.

The tensile strength of the expanded, amorphous-lockied laminate was4300 psi in each direction. Its thickness was 6.4 mils.

EXAMPLE 7 Expanded, Amorphous-Locked Film As A Filtering Membrane Or ASemi-Permeable Membrane

An extruded, calendered, unsintered "Teflon" 6A poly(tetrafluoroethylene) film was made using the known conventionalprocedure described above. This film was expanded and amorphously lockedusing the machine of FIG. 2 and the process of this invention describedherein. Expansion was carried out at a temperature of about 300° C., andamorphous locking at about 370° C. Properties of the original film andthe expanded, amorphous-locked film are listed below:

                  TABLE 9                                                         ______________________________________                                                       Original Unexpanded,                                                                         Expanded                                        Property       Unsintered Film                                                                              Sintered Film                                   ______________________________________                                        Thickness, mils                                                                              4.0            3.5                                             Surface area*, relative                                                                      1.0            2.8                                             units                                                                         Specific Gravity                                                                              1.46           0.60                                           Permeability to air, metric                                                   units          1.0 × 10.sup.-4                                                                         0.032                                          Permeability to kerosene,                                                     metric units   7.0 × 10.sup.-7                                                                        2.3 ×10.sup.-4                            ______________________________________                                         *Length × width                                                    

Smoke-containing air was filtered through a sample of the expanded,amorphous-locked film described above. It was observed that the filteredair was clean, and the filtering rate was relatively high. A similareffort to filter smoke-containing air using a sample of the unexpanded,unsintered film described above was unsuccessful because the filteringrate was too low.

Similarly, samples of the expanded, amorphous-locked film describedabove were used to filter solids from suspensions of the solids invarious organic liquids. Again, good separations were obtained, andfiltering rates were reasonably high. However, similar attempts usingsamples of the unexpanded, unsintered film described above again wereunsuccessful because of extremely low filtering rates.

When an effort was made to flow water through the (air-saturated)expanded, amorphous-locked film described above using 5 psi flowingpressure, no flow occurred. However, when the applied flowing pressureexceeded 10 psi, the water entry pressure of the gas-saturated membrane,flow started, and thereafter flow of water through the membrane wasquite similar to the flow of wetting organic liquids. This membrane wasfound to be useful in separating solids from dispersions of the solidsin water.

A sample of the expanded, amorphous-locked film described above wasfitted into the cone of a filtering funnel, and a mixutre of keroseneand water was poured into the funnel. The kerosene flowed through thefilm at a reasonably rapid rate, but no water penetrated the film sincethe pressure in the water phase was lower than the water entry pressureinto either the gas-saturated or the kerosene-saturated film. Thus, theexpanded, amorphously locked film was found to be an effectivesemi-permeable membrane useful in separating fluids that wettetrafluoroethylene polymers from non-wetting fluids. Similar attemptsto use the unexpanded, unsintered film described above as asemi-permeable membrane were unsuccessful because of the extremely lowflow rates involved.

EXAMPLE 8 Expanded, Amorphously Locked Film Impregnated With Poly(MethylMethacrylate)

A part of the expanded, amorphous-locked film prepared as described inExample 7 was painted with a freshly made solution of 1% ofpolymerization initiator 2,2' azo-bis (2-methylpropionitrile) in methylmethacrylate). The solution was rapidly imbibed into the expanded,amorphously locked film. Any excess solution not so imbibed was wipedfrom the surface of the film.

Then the impregnated film was warmed, causing the methyl methacrylate topolymerize within the pores of the expanded, amorphous-locked film, thusyielding a film having pores filled with poly(methyl methacrylate).

The comparison shown below of the properties of conventional extruded,calendered, unsintered poly(tetrafluoroethylene) film with those of theexpanded, amorphous-locked, film impregnated with the methacrylatepolymer shows clearly the greater dimensional stability of theimpregnated film without significant increase in the coefficient offriction. These properties make impregnated materials of the typedescribed here particularly useful as bearing materials. Thesubstantially lower cost of the impregnated material, as compared withthe conventional homopolymer or copolymers, is a further benefit of thisinvention.

                  TABLE 10A                                                       ______________________________________                                                    Conventional Expanded, Amorphous-                                             Unexpanded,  Locked, Impregnated                                  Property    Unsintered Film                                                                            Film                                                 ______________________________________                                        Deformation, 150                                                              psi Compressive                                                               Stress at 77° F,%                                                                  2.7          0.7                                                  Coefficient of Fric-                                                          tion against Glass                                                                        0.20         0.21                                                 ______________________________________                                    

In further impregnation experiments, a piece of expanded,amporphous-locked poly(tetrafluoroethylene) film made as described inExample 10 was impregnated with a low viscosity epoxy resin, ERLA 2256,a product of and obtainable from Union Carbide Corporation. A secondpiece of the film was impregnated with a solution ofmetaphenylenediamine in methyl ethyl ketone. When the ketone hadevaporated, the two pieces, with the longitudinal dimension of onecoinciding with the transverse dimension of the other, were placed incontact with each other, and the assembly was heated at about 300° F forabout three hours.

The two pieces were firmly bonded by the hardened epoxy resin.Properties of the laminate were as follows:

                  TABLE 10B                                                       ______________________________________                                                      Expanded, Amorphous-                                            Property      Locked Film      Laminate                                       ______________________________________                                        Longitudinal tensile                                                          strength, psi 8,100            8,800                                          Transverse tensile                                                            strength, psi 1,500            8,800                                          Deformation, 100 psi                                                          Compressive Stress                                                            at 77° F., %                                                                         13               1.2                                            Coefficient of Friction                                                       Against Glass 0.14              0.14                                          ______________________________________                                    

EXAMPLE 9 Use Of Expanded, Amorphous-Locked Tape As Core Of A CoaxialCable

Expanded, amorphous-locked tape was made following the proceduredescribed in Example 7. Two such tapes were made, both having a specificgravity of about 0.66, one having a thickness of 2.5 mils, the other, 10mils. Alternate wraps of (1) the thinner tape, (2) the thicker tape, and(3) the thinner tape were used to make up a core separating the innerconductor of the coaxial cable from an outer metallic braided shield. Anouter jacket constructed of conventional poly(tetrafluoroethylene)covered the shield. The characteristic impedance of the cable was 100ohms.

A second coaxial cable having an impedance of 100 ohms was constructed,in this case using conventional, unexpanded tape to construct the core.After sintering, the denisty of the poly(tetrafluorethylene) core wasabout 2.15 gms/cc.

Because of the lower dielectric constant of expanded, amorphous-lockedpoly(tetrafluoroethylene) over that of the conventional polymer, asmaller, lighter cable was obtained when expanded, amorphous-locked tapewas used. This is shown in detail in the following table.

                  TABLE 11                                                        ______________________________________                                                    A             B                                                               100 Ohm       100 Ohm Impedance                                               Impedance Cable                                                                             Cable Made Using A                                              Made using A  Core of Expanded,                                               Core of Conven-                                                                             Amorphously Locked                                  Item        tional Polymer                                                                              Polymer                                             ______________________________________                                        Conductor Weight,                                                                         0.064         0.064                                               g/ft                                                                          Polymer Insulation                                                            g/ft        3.890         0.464                                               Braided Metal                                                                 Shield g/ft 2.700         1.898                                               Polymer Jacket,                                                                           0.855         0.569                                               g/ft.                                                                         Core Diameter, in.                                                                        0.110         0.065                                               Outer Diameter of                                                             Cable, Inch.                                                                              0.140         0.095                                               Total Cable Weight,                                                           g/ft        7.509         2.995                                               ______________________________________                                    

The data listed above show that the use of expanded, amorophous-lockedpolymer in B rather than conventional polymer in A as the core in thiscable led to a 60% reduction in weight and a 32% reduction in size ofthe cable.

EXAMPLE 10 Films Which Are Very Greatly Expanded And ThenAmorphous-Locked

Unsintered, extruded, calendered poly(tetrafluoroethylene) film was madeusing the known conventional procedure described in earlier examples.This film had a thickness of 4.0 mils.

Using the apparatus of FIG. 2 and above procedures, parts of this filmwere expanded without amorphous-locking using a step-wise procedure. Themachine was set at 190% expansion for each of the expansion runs. Thensamples of the expanded films were passed through the machine to lockthem amorphously at 370° C. without further expansion. The stepsfollowed in this work are explained in the following diagram: ##STR1##

Properties of the films produced as described above are listed below:

                  TABLE 12                                                        ______________________________________                                        Film     Expansion,                                                                              Thickness, Specific                                        Identity %         mils       gravity                                         ______________________________________                                        Original film                                                                          none      4.0        1.50                                            Product 1                                                                               190      3.8        0.50                                            Product 2                                                                              190 × 2                                                                           3.8        0.27                                            Product 3                                                                              190 × 3                                                                           3.1        0.18                                            Product 4                                                                              190 × 4                                                                           2.8        0.17                                                               Bulk       Long. tensile                                   Film     Porosity, Long. Tensile                                                                            strength of polymeric                           Identity %         Strength, psi                                                                            matrix, psi                                     ______________________________________                                        Original film                                                                          35        1,640       2,600                                          Product 1                                                                              78        2,900      14,000                                          Product 2                                                                              88        2,420      30,000                                          Product 3                                                                              92        2,400      30,000                                          Product 4                                                                              93        2,400      34,000                                          ______________________________________                                    

EXAMPLE 11

"Teflon" 6A polymer was heated for 3 hours at 300° C., cooled, blendedwith 80 cc of refined kerosene per pound of polymer, and extruded into afilm 6 inches wide, 0.030 inch thick, using a reduction ratio of about100 (reduction ratio = cross-section area of extrusion cylinder dividedby the cross-section of the extrudate). The extruded film was thenpassed through successive sets of rolls, each heated to about 80° C.,and reduced in thickness from 0.030 inch to 0.002 inch. This film wasdried to remove the kerosene and passed through the apparatus of FIG. 2at a rate of 100 ft./min. over roll 15, with rolls 15 and 16 heated to320° C. and adjusted with their outer peripheries as close together aspossible without crushing the 0.002 inch film between them. Roll 16 (and17, 18, 19) was ritated at a peripheral speed seven times greater thanroll 15, thus stretching the film about sevenfold. The film was passedover roll 17 at 370° C. and wound up on take-up 20. Rolls 15, 16, 17, 18and 19 were then adjusted to the same peripheral speed of 30 ft./min.,rolls 15, 16 and 17 adjusted to 370° C., and the stretched film passedthrough the apparatus under these conditions in order to accomplish anadequate heat treating. The properties of the film were as follows:

    ______________________________________                                        Thickness       .0019"                                                        Density gm/cm.sup.3                                                                           .23                                                           Longitudinal tensile psi                                                                      12,200                                                        Longitudinal tensile of                                                       polymer matrix                                                                                 ##STR2##                                                     ______________________________________                                    

EXAMPLE 12 Amorphous Content of Polymer

A sample of film was prepared as in Example 11 except that it was rolledto a thickness of 0.004 inch. This film was then expanded using the sameprocess as in Example 5 except that roll 17 was not heated. Heattreatments were carried out on samples of this film at 335° C., 350° C.,and 390° C. for various lengths of time. The amorphous content of thepolymer was determined at each stage in the process using the infra-redmethod described by Moynihan, R. E. "IR Studies onPolytetrafluoroethylene", J. Am. Chem. Soc. 81, 1045-1050 (1959). Theproperties of the films were as follows:

                  TABLE 13                                                        ______________________________________                                                   Longitudinal                                                                  Tensile Strength/                                                             Matrix     %         Density                                                  Tensile Strength                                                                         Amorphous gm/cm.sup.3                                   ______________________________________                                         "Teflon" 6A powder,                                                          heat treated              1.5%                                                Exturded, dried .004"                                                                      2650         1.5%      1.5                                       film                                                                          Expanded not heat-                                                                         4200/14,200  1.5%      .68                                       treated                                                                       Heated to 335° C:                                                      1 second     5580/18,500  2.5%      .69                                       10 seconds   5630/18,400  3%        .70                                       50 seconds   6020/19,700  4%        .70                                       480 seconds  7540/24,600  5%        .70                                       Heated to 350° C:                                                      1 second     7630/24,700  10%       .70                                       3 seconds    7670/24,900  10%       .70                                       10 seconds   7820/25,200  15%       .70                                       20 seconds   7830/24,800  25%       .70                                       50 seconds   8360/26,400  30%       .70                                       100 seconds  8610/27,100  33%       .70                                       480 seconds  8900/27,900  35%       .70                                       Heated to 390°  C:                                                     1 second     7500/23,500  25%       .71                                       3 seconds    7960/29,900  35%       .73                                       10 seconds   7830/23,400  38%       .73                                       20 seconds   7270/20,300  40%       .78                                       50 seconds   6560/16,800  40%       .85                                       90 seconds   disintegrated                                                    ______________________________________                                    

EXAMPLE 13 High Strength, Low Porosity Films

A sample of expanded but not heat-treated film from Example 12 wasplaced in a platen press, compressed at 300 psi and while heldcompressed, the platens were heated to 350° C. and then cooled rapidly.The longitudinal tensile strength of the resulting film was 24,000 psiand the density 2.10 gms/cm³, about 3% porosity. Therefore, it ispossible to produce very high strength, high density products bycompressing the expanded material during the amorphous-locking step. Thefibril-node structure is preserved even though the porosity is reducedto about 3%. With higher pressures it is possible to further reduce theporosity and still preserve the very high strength of the material.

A second sample of the expanded film from Example 12 which had been heattreated at 350° C. for 8 minutes was placed in the press at roomtemperature and compressed at 1500 psi for several minutes. The film wasclear and transparent. Its density was 2.05 gms/cm³ and longitudinaltensile strength was 21,000 psi. Therefore, it is feasible to compressthe porous structure of the product and still preserve the high strengthof the bulk polymer.

The foregoing examples clearly show the desirable effect of expansionand amorphous-locking on the tensile strength and densitycharacteristics of the products, and also that the high tensile strengthis retained when the porous structure is compressed.

The formation of the porous material by this invention can beaccomplished using poly(tetrafluoroethylene) or copolymers oftetrafluoroethylene with other monomers. Such monomers are ethylene,chlorotrifluoroethylene, or fluorinated propylenes, such ashexafluoropropylene. These monomers are used only in very small amountssince it is preferred to use the homopolymer for the reason that itpresents the optimum crystalline/amorphous structure for the process andthe products of this invention. Thus, amounts of the comonomers aregenerally less than 0.2% and it is highly preferred to usepoly(tetrafluoroethylene). While the above examples show the use ofasbestos as a filler, it is to be appreciated that a wide variety offillers can be incorporated such as carbon black, pigments of variouskinds as well as inorganic materials such as mica, silica, titaniumdioxide, glass, potassium titanate, and the like. Further, fluids may beused which include dielectric fluids or materials such as thepolysiloxane materials shown in U.S. Pat. No. 3,278,673.

While the invention has been disclosed herein in connection with certainembodiments and certain structural and procedural details, it is clearthat changes, modifications or equivalents can be used by those skilledin the art; accordingly, such changes within the principles of theinvention are intended to be included within the scope of the claimsbelow.

What is claimed is:
 1. A process for the production of a porous articleof manufacture of a polymer of tetrafluoroethylene containing a filler,which process comprises:(a) blending a powder consisting essentially ofhighly crystalline poly(tetrafluoroethylene) with a filler; (b)extruding said polymer powder and filler using a conventional lubricatedextrusion technique to form an extrudate; (c) removing the lubricantfrom said extrudate by conventional methods; and (d) stretching saidextrudate containing unsintered poly(tetrafluoroethylene) and filler ata rate exceeding about 10% per second, said stretching being performedwhile said extrudate is held at a temperature of about 35° C to 327° C.2. The process of claim 1 in which said filler is asbestos.
 3. Theprocess of claim 1 in which said filler is carbon black.
 4. The processof claim 1 in which said filler is pigment.
 5. The process of claim 1 inwhich said filler is mica.
 6. The process of claim 1 in which saidfiller is silica.
 7. The process of claim 1 in which said filler istitanium dioxide.
 8. The process of claim 1 in which said filler isglass.
 9. The process of claim 1 in which said filler is potassiumtitanate.
 10. The process of claim 1 in which said filler is adielectric fluid.
 11. The process of claim 1 in which said filler is apolysiloxane.
 12. The process of claim 1 in which said stretching isperformed uniaxially.
 13. The process of claim 1 in which saidstretching is performed biaxially.
 14. The process of claim 1 includingthe subsequent step of heating said porous, stretched, filled article toa temperature above the crystalline melting temperature of said polymer.